Various proposals have been made for circulating cooling air from the compressor of a gas turbine engine substantially entirely about an annular combustor associated with a radial turbine wheel. In the usual case, the cooling air stream is confined against the combustor by a plenum such that it flows first about the radially outer wall of the combustor, then about a generally radial wall of the combustor oppositely from the combustor outlet, and then about a radially inner wall to ultimately impinge against the rear turbine shroud and be directed somewhat radially outwardly to discharge at the turbine nozzle through a space between an edge of the combustor and an edge of the rear turbine shroud.
While this approach works well, in some instances, certain difficulties present themselves. For example, it is not unusual to start gas turbines by rapidly accelerating the turbine wheel with a stream of hot gas derived from a pyrotechnic source, frequently referred to as a start cartridge. While the means by which such hot gas is directed against the turbine wheel take on a number of forms, it is not uncommon to provide one or more apertures in the rear turbine shroud along with associated piping on the side of the rear turbine shroud opposite from the turbine wheel to the start cartridge.
Such piping is, of course, located in the path of cooling air flowing about the combustor and is a non-unif,orm structure in such space, providing non-uniform resistance to the flow of cooling air. Non-uniform resistance results in non-uniform cooling air flow which in turn results in nonuniform cooling. Non-uniform cooling results in the generation of thermal stresses during operation of the turbine and that in turn will ultimately reduce the life of the engine.
To avoid this problem, it has been customary to cover the piping or the hot gas starter nozzle with an aerodynamically uniform cover. The cover of course adds to the cost as well as the complexity of the engine.
The present invention is directed to overcoming the above problem.